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Assessment of Solar System Exploration Programs 1991 (Chapter 3) Assessment of Solar System Exploration Programs 1991 3 Exploration of the Outer Planets The committee's first science strategy for outer planet exploration was published in Report on Space Science 1975 (SSB, 1976). That strategy was reviewed and revised in A Strategy for the Exploration of the Outer Planets: 1986- 1996 (SSB, 1986b). The principal science goals and objectives and related recommendations follow. SCIENCE OBJECTIVES The Saturn System REPORT MENU The highest priority for outer planet exploration in the next decade is NOTICE intensive study of Saturn—the planet, satellites, rings, and magnetosphere—as a MEMBERSHIP system. FOREWORD SUMMARY CHAPTER 1 Specifically, the recommended exploration and intensive study of the CHAPTER 2 Saturn system includes the following objectives: CHAPTER 3 CHAPTER 4 Titan's atmosphere—measure the composition, structure, and CHAPTER 5 CHAPTER 6 circulation of Titan's atmosphere, and characterize the atmosphere-surface CHAPTER 7 interaction. CHAPTER 8 CHAPTER 9 Titan's surface—carry out a reconnaissance of the physical properties REFERENCES and geographical variability of Titan's surface: solid or liquid, rough or smooth. Emphasis should be given to any information needed to guide the design of a lander vehicle. Saturn's atmosphere—determine the elemental composition, dynamics, and cloud composition and structure, to a level well below the H2O file:///C|/SSB_old_web/ssep91ch3.htm (1 of 5) [6/18/2004 1:57:44 PM]

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Assessment of Solar System Exploration Programs 1991 (Chapter 3) cloud base. Saturn's rings—measure particle composition and its variety, spatial distribution of particles, and determine the evolution of dynamic structures. Saturn's small satellites—make comparative determinations of surface composition, density, geologic history, and geomorphological processes. Saturn's magnetosphere—specify the structure, dynamics, and processes, and the material interactions of the magnetosphere with Saturn's atmosphere, rings, icy satellites, Titan, and the solar wind. The Outer Solar System The next priority for outer planet exploration is assigned to post-1996 objectives that depend on the results of current missions (analysis of the Voyager data and exploration by the Galileo mission), developments in instrumentation, and demonstrations of technical feasibility. Specifically, these longer term objectives include exploration and intensive study of the following: Uranus and Neptune systems—(1) elemental composition, cloud structure, and meteorology of the planetary atmospheres; (2) rings and satellites (especially Triton); and (3) structure and dynamics of magnetospheres. Planetology of the Galilean satellites and Titan—surface composition and physical properties, seismic activity, heat flow, and where applicable, atmospheric composition and meteorology. Inner Jovian system—density, composition, and energy of magnetospheric particles; large-scale structure, rotation, and time-dependent phenomena in the Io torus, and relation to Io and other satellites, orbiting gas and plasma, auroral activity on Jupiter, and electromagnetic emissions. Pluto, the only known planet in the solar system as yet unvisited by spacecraft, will continue to be an important target for Earth-orbital and Earth- based studies. As a goal for the long term, a Pluto flyby or orbiter is clearly of great interest. CURRENT STATUS OF NASA'S EXPLORATION OF THE OUTER PLANETS The 1986 COMPLEX report contains two major recommendations. The file:///C|/SSB_old_web/ssep91ch3.htm (2 of 5) [6/18/2004 1:57:44 PM]

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Assessment of Solar System Exploration Programs 1991 (Chapter 3) first is: "The highest priority for outer planet exploration in the next decade is intensive study of Saturn—the planet, satellites, rings, and magnetosphere as a system." The proposed joint NASA/ESA Cassini mission, planned to arrive at Saturn in 2002, constitutes a major step toward achieving this goal. This mission would enable intensive study of Titan's atmosphere and surface, and Saturn's rings, small satellites, and magnetosphere. The Cassini orbiter and probe investigations from the U.S. and European scientific communities were recently selected, and instrument development continues. The report's second major recommendation involved post-1995 objectives for the exploration and intensive study of the Uranus and Neptune systems, the planetology of the Galilean satellites and Titan, and the inner Jovian system, whose implementation depends on the results of the Voyager and Galileo missions. The committee is pleased to note that both missions have proceeded as planned. Voyager 2 completed its reconnaissance of the outer planets upon its encounter with Neptune in August 1989. Analysis of the Neptune data has yet to be completed. Galileo flew by Venus and Earth in 1990, and the spacecraft appears to have adequate fuel reserves for two asteroid encounters and a full- term petal orbit for studying the Jupiter system. During the five-year period ending in 1990, several events have increased both knowledge of and interest in the Pluto-Charon system: (a) a series of Pluto- Charon mutual eclipses, (b) discovery of Pluto's atmosphere by Earth-based observations of a stellar occulation, and (c) analysis of IRAS serendipitous observations of Pluto. The Hubble Space Telescope also has produced a spectacular image of the Pluto-Charon system. Given our enhanced understanding of the system, the committee recognizes that a well-conceived spacecraft mission to Pluto would be a significant contribution to the scientific goals of planetary exploration. No progress has been made on one major part of the committee's highest priority for outer planets exploration, namely, exploration and intensive study of Saturn's atmosphere. The committee's 1986 report specified determination of the elemental composition, dynamics, and cloud composition and structure to a level well below the H2O cloud base. To meet these objectives will require an atmospheric probe that can go deeper than the Galileo probe. The report called for studies of such deep atmospheric probes, and these studies should be pursued as soon as possible. Study of deep atmospheric probes is just one of a variety of support activities that the committee's 1986 report recommended as being necessary for meeting the near-term and long-term objectives for exploration of the outer solar system. These support activities remain a necessary and integral part of a scientifically balanced exploration strategy. In addition to the studies of deep atmospheric probes, other areas where little progress has been made are as follows: Support of laboratory and theoretical studies. The committee supports file:///C|/SSB_old_web/ssep91ch3.htm (3 of 5) [6/18/2004 1:57:44 PM]

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Assessment of Solar System Exploration Programs 1991 (Chapter 3) the initiation of the Planetary Instrument Upgrade Program (PIUP) at the earliest possible date. Development of radiation-hardened spacecraft. This has not been pursued at a level sufficient to make significant progress. Development of low-thrust propulsion systems. This area of technology development has likewise not received significant support. Last update 12/13/00 at 8:20 am Site managed by Anne Simmons, Space Studies Board file:///C|/SSB_old_web/ssep91ch3.htm (4 of 5) [6/18/2004 1:57:44 PM]